Gen 1 was a nice and simple cave that had at the time the best Pokemon ever in it. Not bad, but thematically nice, especially since the game built it up if you went to Cerulean Cave. FR/LG added the last of the caves which was good fun and provided many new Pokemon you couldn't get before.

Gen 2 was a fantastic surprise, but ultimately let down by the fact that so much of the region was gutted. HG/SS remedied this, but the increased levels turn an already awful grinding experience into a slog and all the HMs were terrible.

Gen 3's Battle Frontier was a lot of fun, the tutors were an unexpected joy, and the challenges were mostly solid, to say nothing of a neat Gen 2 reference and caves that had two of the most important Pokemon for breeding in ready supply. This is a bit brought down by the awfulness that is the Battle Pyramid and the fact that you inevitably found a random wild shiny that you couldn't catch because it was part of the challenge. The Frontier's challenges were also not always implemented well: the Pyramid was a slog, the Palace was all RNG, the Arena felt arbitrary in many ways...

Gen 4's Battle Frontier was solid and helped mitigate a lot of the issues with the original BF, but the removal of certain challenges was a bad idea (I'm looking at you, Dome). The Castle was a nice strategic location and the Arcade's random element was still in your control. The only real issue was the questionable Hall.

5 gave us a place that made grinding for exp and money a breeze (especially with the implementation of Pass Powers), a nostalgia marathon with the PWT (and without the gutting of Gen 2), and it expanded the most core of Battle Frontier experiences (if only from their perspective). We also got to explore the rest of a sizable region and wrapped up the story to its completion.

I want to say that Gen 5 gave the best locations, though I concede that it feels like a continuation of Gen 3 and 1.---"Murder of the living is tragic, but murder of the idea is unforgivable." - Janus, speaker of the synod

I don't consider Kanto to be part of the Post game of Johto, so I'm going with Black and White here. It fixes all the critical flaws the main game had.---Not changing this sig until we get a new main series Tales game released on a Nintendo console in the US